Key Equations

Energy, Power & Resistance - OCR A-Level Physics

On Data Sheet
Not on Data Sheet
Resistance
$$R = \frac{V}{I}$$
  • Where:
    • $R$ = ohm
    • $V$ = V
    • $I$ = A
Definition of resistance. Applies to ALL components, not just ohmic conductors.
Resistivity
$$\rho = \frac{RA}{L}$$
  • Where:
    • $rho$ = ohm m
    • $R$ = ohm
    • $A$ = \(m^{2}\)
    • $L$ = m
rho is a material property. Always convert diameter to area in \(m^{2}\) first.
Power (current x voltage)
$$P = IV$$
  • Where:
    • $P$ = W
    • $I$ = A
    • $V$ = V
Fundamental power equation. Other forms derived from this + Ohm's law.
Power (current squared x resistance)
$$P = I^{2}R$$
  • Where:
    • $P$ = W
    • $I$ = A
    • $R$ = ohm
Useful when V is unknown. Shows that doubling current quadruples power.
Power (voltage squared / resistance)
$$P = \frac{V^{2}}{R}$$
  • Where:
    • $P$ = W
    • $V$ = V
    • $R$ = ohm
Useful when I is unknown. For components in parallel (same V), lower R means more power.
Energy transferred
$$W = IVt$$
  • Where:
    • $W$ = J
    • $I$ = A
    • $V$ = V
    • $t$ = s
W is energy in joules, t is time in seconds. Also written as E = Pt.
Accelerating charged particles
$$eV = \frac{1}{2}mv^{2}$$
  • Where:
    • $e$ = C
    • $V$ = V
    • $m$ = kg
    • $v$ = m/s
Energy gained by charge e through PD V becomes kinetic energy. Assumes particle starts from rest. Combine E_k = ½mv² (on data sheet) with eV definition - this combined form is not a single data sheet equation.
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